These games have tens of millions of lines of code (often building on top of the older components). They are also written in memory unsafe C++, by developers focused on features and rapid sales, to users that look forward to new features not better security. I won’t be surprised at all if they are filled with vulnerabilities.
Uhhh, GTA V has been known for bad security for a long time. Streamers can't even play the game without a VPN or they get their IP leaked. Single-player games can (could, maybe still can? I am unsure of the current state of the game) get hacked and be messed with by hackers. This goes far beyond just "they used C++." This shows a great lack of caring for security in general from Rockstar, not just another thing that you can use to prove to yourself that "C++ is dangerous!"